1. Field of the Invention
The present invention relates to a flow rate control device for use in relief cut-off control in a hydraulic excavator.
2. Description of the Related Art
In a certain hydraulic excavator there is made a control called relief cut-off control in which the discharge flow rate from a pump is decreased when the pump pressure has reached a relief cut-off pressure. Once this relief cut-off control is made, for example when a rock or the like strikes against a bucket during excavating work of the hydraulic excavator and an overload acts on the excavator, it is not only possible to prevent the energy loss of hydraulic oil caused by relief operation but also possible to diminish the relief noise. Besides, there does not occur any shock during relief operation, so it becomes possible to effect smooth operation and it is also possible to improve the heat balance of hydraulic oil and devices.
In the conventional hydraulic excavator, however, the relief cut-off control is made on the basis of only a preset relief pressure of a relief valve. That is, the relief cut-off control is made irrespective of what work and under what conditions the hydraulic excavator is performing.
Generally, it is known that once the relief cut-off control is made, the operator of the hydraulic excavator feels a sense of incongruity or feels as if force were gone, as an inevitable result of decrease in the discharge flow rate from the pump.
In the above conventional hydraulic excavator, the relief cut-off control is made each time the load imposed on a hydraulic actuator increases to a certain degree. Consequently, at every relief cut-off control, a sense of incongruity or an extinct-of-force feeling is given to the operator. As the case may be, it may become impossible to continue operation due to an insufficient pump discharge.